Access to clean water is essential to the ability of mankind and the environment to survive and thrive. The initial step to cleaning contaminated and non-contaminated water is the separation and removal of suspended solids from water, a method referred to as dewatering.
Currently, there is a need to separate solids from liquids and separate liquids from other liquids more efficiently, effectively and economically, in large volumes, at higher speeds, and with a more compact environmental footprint. At the same time it is desired for separation techniques to have less equipment, reduced energy requirements and reduced pollution; reduced manpower as well as a reduced disruption to commerce and the community.
There are numerous examples during recent history where current separation methods were incapable of responding or preventing emergency situations and disasters. The emergencies and disasters resulted in a severe impact on waterways, the environment and the local economies surrounding the events. Examples of such devastating events include oil spills, such as the 2010B P Gulf oil spill; pipeline breaches, and the 2010 Kalamazoo River oil spill; tailing ponds and sludge lagoon breaches, such as 2014 Duke Energy fly ash spill and the 2008 TVA Kingston coal ash spill; Flooding, such as the 2015 Mississippi River flooding and 2015 Oregon flooding; and unintended releases, such as the 2015 EPA Colorado River Gold King.
Currently, operations requiring slurry dewatering or liquid-liquid separation employ a series of mechanical systems, technologies, pumps and conveying mechanisms for primary dewatering and liquid-liquid separation followed by the conveyance of any resulting sludge to downstream mechanisms in order to sufficiently perform secondary or tertiary dewatering and dehydration of the resulting sludge. Such secondary and tertiary methods in the current state of the art are needed to provide economical transport, disposal, asset recovery or beneficial reuse. Examples of these operations include: dredging operations, mining operations, paper mills, gas and oil operations, including fracking and oil sands, oil spill cleanups, tailing ponds and sludge lagoons, sewage, septage and wastewater treatment, confined animal feeding operations (CAFO), food processing, soil and cake washing, nutrient removal and reduction and desalination.
The most widely employed technologies and methodologies for primary dewatering are settling clarifiers and detention settling ponds or lagoons. Secondary and tertiary dewatering and sludge dehydration typically employ belt filter presses, plate and frame filter presses, centrifuges, rotary presses, dewatering cells or boxes, or geo-textile bags. All of these technologies and methodologies are bulky and heavy, requiring a significant energy footprint to support operations, including time and energy. Moreover, widely employed technologies are manpower intensive. Due to the weight and footprint requirements, it is impractical to combine or stack multiple technologies onto a single compact vertical footprint.
The previously employed systems, technologies, processes and equipment, which usually operate and function separately and independently, are not easily transportable due to being large and heavy. Conveying slurry and sludge between available systems typically requires pumps, piping, fittings, spill containment, fuel or other energy resources, manpower and operator attention. In order to be transported, setup and teardown the equipment, a substantial operational footprint and related support structures are also required, as well as significant time, manpower, heavy equipment, and extensive resources to prepare and remediate the operating site.
For centuries, the settling of suspended solids, and then decanting the supernatant in confined vessels or basins, has been employed to separate solids and water, commonly referred to as settling clarification. Settling basins are sized according to slurry flow rates and the suspended solids settling or hydraulic retention time (HRT) required on a direct correlation basis. As an example, for each 500 gallons of water requiring 15 minutes of settling or HRT, a vessel capable of containing 7,500 gallons (500 gallons times 15 minutes) and sturdy enough to hold at least 62,550 pounds (7,500 gallons of water weighing 8.34 pounds per gallon) in addition to the volume and weight of the sedimented solids would be required. At some point, either the buildup of sediment must be removed or additional vessels employed.
Dewatering is accomplished in nature through the gravity settling of suspended solids in a water column. Then, as water flows through and around natural filter medias, such as gravel, sand and vegetation, additional suspended solids are separated and removed. This process, though often quite effective, takes considerable time, often months, years, decades or longer. As the world's population grows, the need to effectively shorten this natural process has become increasingly important. Man's pollution of water due to farming and industrial operations, has greatly impacted and complicated clean water issues.
The advent of slurry conditioning substances, such as alum, ferric and polymeric reagents, which encourage suspended solids to agglomerate into larger and heavier masses, commonly referred to as “flocs” in order to enhance settling, provided a solution to shorten settling time. However, slurry conditioning often created additional volume of settled solids, and did not adequately address floating, indefinitely suspended solids or re-suspended solids in a water column. As the basins filled with sediment, they were either abandoned and new basins were employed, or the sediment was removed. Both of these options resulted in significant amounts of saturated, high moisture content and low viscosity sediment, which was not easily transportable.
Several types of clarifiers have been employed. The principal is simple. Influent slurry is pumped into a vessel or confined area, allowing the suspended solids to gravitationally settle or fall out of the water column and then sediment over time. Lamella clarifiers employ inclined plates, often in a parallel fashion. Slurries are normally conditioned, pumped or forced up the plates. The settling sludge comprised of flocs and suspended solids, may sediment onto the inclined plates. When sufficient aggregation of sludge on the plates occurs, a laminar flow forms, allowing the settling sludge mass to more easily gravitationally flow down the plates.
The rate of settling is directly dependent and correlated to the weight and mass of the suspended solids, as expressed by Stoke's Law and the Ferguson and Church Equation.
However, both Stoke's Law and the Ferguson and Church Equation are based on uniform spherical shaped material, which is rarely, if ever, the case. Even with the introduction of conditioning reagents, many ultra-fine solids or substances that are difficult to settle may remain in a water column and require extended HRT and therefore may require additional clarifiers to handle slurry flow rate requirements. There are several substances or particles with a low specific gravity, but with a significant mass, that may take many seconds, minutes, hours, days or longer to settle. Many substances even when conditioned, such as ultra-fine solids and colloids, may remain indefinitely suspended, may float or may become suspended again with minimal turbulence. Under conventional methods, if a slurry requires a prolonged HRT, either the throughput rate of the system may be lowered or the employment of additional clarifiers are required.
Once the suspended solids have sufficiently settled, the top layer of the water column, or supernatant, is then available to be decanted over a weir and discharged as effluent. The discharged effluent is often referred to as “free water” or “primary water” separation, or primary dewatering. Suspended solids or flocs that do not settle, such as those that remain indefinitely suspended, re-suspended or float, will typically remain in the supernatant and be discharged along with the decanted effluent.
Due to sedimentation and HRT, the suspended solids which do sediment at the bottom of the clarifier may benefit somewhat from compaction, but are typically a saturated or water laden mass having low viscosity. This is typically due to the formation of “sludge blankets” where layers of settling suspended solids loosely bond, then settle as a blanket. Water then becomes trapped between blanket layers creating additional interstitial or capillary water in the sedimented sludge, rendering the sedimented sludge difficult to handle, due to lower viscosity, and typically must be pumped from the bottom of the settling clarifier. The sedimented sludge is then conveyed, typically by pumps, to downstream mechanisms for additional dewatering or dehydration. Due to HRT requirements for processing, settling clarifiers typically require a large footprint and are very heavy, as they must be rigidly constructed out of strong materials to not only support the volume of slurry, but also the accumulation of settled or sedimented solids. Consequently, due to size and weight, settling clarifiers capable of processing slurry flow rates exceeding 400 GPM are not easily transportable, as they usually exceed the road height and width restrictions. Clarifiers capable of handling slurry flow rates exceeding 400 GPM typically take extended time, manpower and heavy equipment, such as cranes, and other resources to disassemble, transport and then reassemble. These clarifiers typically require setup and placement on a reinforced and supported footprint.
TABLE 1Technology Required to Process SlurryFootprintFootprintwith 10% TSS by Volume and 1.5SquareCubicEmptyOperationalSpecific GravityQuantityWidthHeightLengthFeetFeetWeightWeightRotary Press10Feet/Pounds15.66.324.5382.22,388.829,16241,662ChannelsMeters/Kilograms4.81.97.535.567.613,22818,898Belt Filter Press2Feet/Pounds13.07.022.4291.22,038.422,04627,7563 Meter BeltMeters/Kilograms4.02.16.827.157.710,00012,590Plate & Frame Filter Press349Feet/Pounds8.56.0119.51,015.86,094.5313,518362,9062000 mm PlatesMeters/Kilograms2.61.836.494.4172.6142,209164,611Dewatering Cells1251.06.340.02,040.012,750.081,120909,72730 Cubic Yard @ 24 Hour HRT15.51.912.2189.5361.024,725412,645Geo-Textile Bags122.56.095.02,137.512,825.06,413900,251HRT6.91.829.0198.6363.22,909408,347Lamella Settling Clarifier1Feet/Pounds11.912.319.2228.52,810.337,000102,67815 Minutes HRTMeters/Kilograms3.63.75.921.279.616,78346,574Dynamic Filtration Clarifier (DFC) &1Feet/Pounds4.06.06.024.0144.01,3312,874Nested-filter Dewatering Cell (NDC)Meters/Kilograms1.21.81.82.24.16041,304Compaction Filter Press (CFP)1Feet/Pounds4.06.06.024.0144.01,8802,565Meters/Kilograms1.21.81.82.24.18531,163DFC-NDC-CFP System Combined1Feet/Pounds4.012.06.024.0288.03,2115,439Meters/Kilograms1.23.61.82.28.21,4562,467
Sludge dewatering and dehydration technologies, such as plate and frame filter presses and belt filter presses apply continuous and increasing pressure to dewater and dehydrate sludge. Belt filter presses position sludge, which has typically thickened, between two filter belts that compress the sludge. Care must be taken not to place an excess amount of sludge between belts or excess pressure on the sludge, as the sludge will be squeezed out the sides of the belts, which cannot be sufficiently enclosed, and therefore reduced or minimal dewatering results will be realized. The belts pass through a path of several sets of paired rollers, with each consecutive set having a reduced spacing between the rollers, thereby each set of rollers exert increased pressure on the sludge between the filter belts. Plate and frame filter presses receive a slurry, (typically conditioned), that is pumped into multiple filter media covered cavities enclosed between opposing plates. Pumping pressure is increased in order to force the suspended solids or “flocs” to the filter and the water to filtrate through filter media.
Plate and frame presses operate in a batch fashion, with each batch cycle requiring multiple hours. Due to the static confinement between two belts or within an enclosed cavity, sludge dehydrates from the exterior to the interior of the sludge mass, trapping or confining interstitial water within the core of the sludge mass. As pressure is increased, interstitial water attempting to escape through capillaries in the sludge mass pushes finer particles, in turn causing a buildup of particles either within the capillaries or into the pores of the filter media. This buildup results in the clogging or collapsing of capillaries or blinding of the filter media.
Applying additional pressure on the sludge mass in an effort to extract additional interstitial water may lead to results with diminishing returns as additional or prolonged pressure eventually clogs or collapses capillaries, or blinds filter media, either inhibiting or blocking the interstitial water's discharge or filtrate path.
Dewatering cells are comprised of a vessel having walls and floors covered with filter media placed over sub-walls and sub-floors that facilitate the discharge of filtrated water. Geo-textile bags are tube shaped vessels enclosed with geo-textile filter media and having fill portals. Partitions or interior walls covered with filter media have also been employed in cells and bags to enhance dewatering by shortening the distance that interstitial water must travel for filtration. A slurry, typically conditioned, is pumped into the cells or bags through fill portals. Natural attenuation, compaction and consolidation of the sludge eventually breach the pore water pressure or tension, forcing interstitial water through capillaries in the sludge and towards filter media for filtration. In cells and bags, aside from contraction of the sludge mass due to water release and discharge, the sludge remains in a static position and is constrained within the boundaries of the cells, partitions or bags, thus restricting sludge movement, repositioning or reconsolidation. This lack of movement and repositioning may lead to capillary collapse as well as clogging and blinding of the filter media. Due to the somewhat static state, cells and bags tend to dewater from the exterior to the interior of the sludge mass, creating a crust or shell of dehydrated sludge on the perimeter of the sludge mass. Crusting impedes interstitial water release through capillaries in the sludge, leaving a saturated, higher moisture content area in the core of the sludge mass, and therefore the sludge is not uniformly dewatered or dehydrated.
Vacuum is sometimes employed to enhance sludge dehydration. However, cracks or cavities formed in the sludge resulting from dewatering and the expression of vacuum eventually creates voids.
As water evacuates sludge, the mass contracts away from the filter media and sources of vacuum, causing cracks and voids to form. Cracking leads to loss of vacuum as the voids fill with air. Sufficient dewatering time is typically many hours or days in the case of cells, or many days, weeks or months in the case of geo-textile bags. Once the sludge has sufficiently dewatered, the resulting cake is evacuated from the cells by opening one of the end or side walls and tipping the cell to evacuate the cake, or by using a mechanism, such as an excavator to extract the cake. Since cells typically employ sub-floors for filtrate plumbing, gravity evacuation of sludge or cake through the floor or bottom of the cell is not practical or possible. Should multi-sided or other irregularly shaped partitions be positioned in the cell, the partitions must be removed from the cell prior to tipping or excavation of the cake. In the case of geo-textile bags, the bags are cut open, the cake is excavated, and the bags are then discarded, as they are not normally reusable.